CHAP, iv.] RENAL SECRETION. 405 



blood-pressure sufficient to account for the flow on the nitration 

 hypothesis. A very similar result is illustrated by the common 

 experience that the flow of urine is largely increased after taking 

 fluids especially in large quantities. We cannot explain this by a 

 reference to blood-pressure, since we have seen (p. 225) that the 

 quantity of blood may be increased largely without raising the 

 blood-pressure. On the other hand, observations with the on- 

 cometer have shewn us that the kidney is remarkably sensitive 

 to changes in the chemical constitution of the blood, an expansion, 

 preceded or not by a passing constriction, being caused by the 

 injection into the blood of even small quantities of water, sodium 

 chloride and other substances. We have further seen that the 

 expansion of the kidney, or rather the dilation of the renal vessels 

 which is the cause of that expansion, brought about in this way, 

 is dependent on a local peripheral action of some kind or other, 

 since it will take place after complete severance of all the renal 

 nerves. It is of course open for us to suppose that this very 

 dilation of the renal vessels is the cause of the increased flow at 

 the same time, since, the general blood-pressure remaining the 

 same, it will lead to increased pressure in the glomeruli. So that 

 the activity of the kidney which follows upon food and drink or 

 upon the injection of urea and other substances into the blood may 

 be taken as really illustrating the dependence of the flow of urine 

 on blood-pressure, though the vascular mechanism concerned is 

 limited to the kidney itself and variations of the general blood- 

 pressure play no part in the matter. But it is also open for us to 

 suppose that the presence of these substances in the blood excites 

 the renal epithelium cells to an unwonted activity, causing them- 

 to pour into the interior of the tubules a copious secretion, just 

 as the presence of pilocarpin in the blood will cause the salivary 

 cells to pour forth their secretion into the lumen of their ducts; 

 and that this activity of the epithelium cells is accompanied, also 

 as in the case of the submaxillary and other glands, by a vascular 

 dilation which, though adjuvant and beneficial, is not the distinct 

 cause of the activity. That this latter view is probably the true 

 one is shewn by the following remarkable experiment, from which 

 we learn that of the various substances finding their way into the 

 blood, some pass into the urine through the glomeruli while others 

 are distinctly secreted by the tubuli uriniferi, their discharge being 

 accompanied by an activity of the secreting cells indicated by the 

 flow of water taking place at the same time. 



In the amphibia, the kidney has a double vascular supply : it 

 receives arterial blood from the renal artery, but there is also 

 poured into it venous blood from another source. The femoral 

 vein divides at the top of the thigh into two branches, one of 

 which runs along the front of the abdomen to meet its fellow 

 in the middle line and form the anterior abdominal vein, while the 

 other passes to the outer border of the kidney and branches in the 



